This invention relates to a method and apparatus for descaling metal strip by pickling. More particularly, this invention relates to a method and apparatus for eliminating the galvanic potential between the metal strip and rolls in the descaling media to improve metal strip surface quality.
In the production of metal alloy strip, the material is subjected to a hot-rolling operation which produces scale on the strip surface. Prior to subsequent cold rolling to final gauge, this scale produced during hot rolling must be removed, otherwise, the scale will be rolled into the strip surface to render the surface quality of the cold-rolled strip unacceptable for the desired final product applications.
It is known to remove this scale after hot rolling by continuously passing the strip through a tank containing a caustic descaling media, such as a molten salt bath. The strip is maintained submerged within the molten salt bath by the use of metal rolls which are likewise at least partially submerged within the molten salt bath.
In molten salt descaling operations of this conventional type, electrical arcing results between the strip and the roll due to the electrical potential difference which exists between the metal strip and the common iron or steel roll. The galvanic potential results in an electrical discharge or arc between the metal strip and roll. The potential difference between the metal strip and rolls is the open cell potential which is discharged through the metal strip and roll contact in the form of electrical arcing. Though potential differences may result for metal strip of steel and stainless steel compositions, it is particularly acute for a class of metal alloys, such as titanium, titanium base alloys, zirconium and zirconium base alloys, which exhibit more dissimilarity from the steel rolls. This arcing results in the production of surface defects on the metal strip after descaling, and these defects render the product less acceptable for its intended final product applications. The defects resulting from the electrical arcing are in the form of pitting of the surface, commonly referred to as electrolytic or galvanic pits.
It is known in the art to reduce or eliminate the galvanic potential between the strip and the roll by the use of a countercurrent electromotive force (EMF) as disclosed in U.S. Pat. No. 2,826,539, issued Mar. 11, 1958. Generally, the countercurrent EMF is useful to reduce electrolytic pitting; however the method requires special techniques and an additional electrical system to produce the EMF. It is also known that the galvanic potential may be reduced by using caustic molten salt bath compositions which permit lower operating bath temperatures. Such caustic baths generally are more expensive and add to overall production costs.
It is, accordingly a primary object of the present invention to provide an improvement in molten salt descaling of metal strip product, particularly titanium and titanium base alloys, wherein surface defects resulting from electrical arcing between the strip and the rolls used to maintain the strip within the molten salt bath are eliminated.
This and other objects of the invention as well as a more complete understanding thereof may be obtained from the following description and specific examples.